Peculiar Damage Mechansim

[jeevesme]

Guys,
I just finished looking at an "Adjustment" Tank that has some peculiar "etching' that is happening on the lower portion. The only pressure on it, is the jacket at the bottom of the tank where the etching is taking place. I talked with the operators and they said they use the tank for mutiple purposes. They manually add (using the top hatch) salt, citric acid, sodium suphate, and sodium bicarbonate to name a few. They typically keep it under 130F. The pH ranges from 4-13 depending on what it is being used for. I have attached a link for some pictures.
When I first learned what the vessel was used for, I immediately thought Chl-SCC. However, we could not get a good PT examination on it because it could not be cleaned adequately to confirm. Also, it has the feel of very fine sandapaper. Similar to emry cloth. I know most likely it is a combanation of damage mechanism. Can anyone shed some light?

 

[jeevesme]

I forgot to add, that the shell welds are much lighter than the rest of the vessel. I assume it has something to do with the welding process. The base metal is 316SS.

 

[EdStainless]

I don't see anything that scares me.
Yes, they could crack it if they add salt at low pH.
The pictures just look like superficial etching.
If you soak 316 in sulfuric acid or nitric acid long enough (or hot enough) you will get a textured surface.
I would lightly sand some of the welds and try to get PT on them.
You could be eating the ferrite out of the welds and making them porous.


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